In law, desuetude (; from French désuétude, from Latin desuetudo 'outdated, no longer custom') is a doctrine that causes statutes, similar legislation, or legal principles to lapse and become unenforceable by a long habit of non-enforcement or lapse of time. It is what happens to laws that are not repealed when they become obsolete. It is the legal doctrine that long and continued non-use of a law renders it invalid, at least in the sense that courts will no longer tolerate punishing its transgressors.

The policy of inserting sunset clauses into a constitution or charter of rights (as in Canada since 1982) or into regulations and other delegated/subordinate legislation made under an act (as in Australia since the early 1990s) can be regarded as a statutory codification of this jus commune doctrine.